Measuring system employing a sensing loop and a reference loop in bucking relationship to provide current-to-voltage conversion

ABSTRACT

This invention is a current-to-voltage conversion system. The use of the conversion system is in measuring AC or DC signals and in sound reproduction systems. The DC conversion system comprises a reference loop and a sensing loop connected in current bucking relation. A transistorized &#39;&#39;&#39;&#39;zero junction circuit&#39;&#39;&#39;&#39; senses the current differential between the two loops and generates an output voltage that is related to the current differential. The DC conversion system is used to sense current changes for a measuring instrument. the AC conversion system is used to sense output current flow in an audio reproduction system and to provide a feedback voltage directly related to the current flow, for example.

United States Patent Wang Oct. 24, 1972 [54] MEASURING SYSTEM EMPLOYINGA 934,596 9/1909 Conrad ..324/140 X SENSING LOOP AND A REFERENCE2,059,786 11/1936 Gilbert ..324/99 LOOP IN BUCKING RELATIONSHIP 2,762,

TO PROVIDE CURRENT-TO-VOLTAGE CONVERSION Chien San Wang, 1201 Hudson,Denver, Colo. 80220 Filed: I Jan. 23, 1970 Appl. No.: 8,758

Related US. Application Data Division of Ser. No. 639,534, May 18, 1967,Pat. No. 3,542,952.

inventor:

References Cited UNITED STATES PATENTS 1/1889 Hayes et al. ..324/98 9/1956 Conant ..32/98 Primary ExaminerRudolph V. Rolinec AssistantExaminer-Emest F. Karlsen Attorney-Duane C. Burton [5 7] ABSTRACT Thisinvention is a current-to-voltage conversion system. The use of theconversion system is in measuring AC or DC signals and in soundreproduction systems. The DC conversion system comprises a referenceloop and a sensing loop connected in current bucking relation. Atransistorized zero junction circuit senses the current differentialbetween the two loops and generates an output voltage that is related tothe current difierential. The DC conversion system is used to sensecurrent changes for a measuring instrument. the AC conversion system isused to sense output current flow in an audio reproduction system and toprovide a feedback voltage directly related to the current flow, forexample.

8 Claims, 7 Drawing Figures PATENTED I972 3.701. 014

SHEEI 1 OF 2 PATENTEU 24 I972 3.701.014

sum 2 or 2 B FIGS mo Cl C I c-ro v D CONVERTER A r AMPLIFIER FIG. 6

MEASURING SYSTEM EMPLOYING A SENSING LOOP AND A REFERENCE LOOP INBUCKING RELATIONSHIP TO PROVIDE CURRENT-TO- VOLTAGE CONVERSION This is adivision of application Ser. No. 639,534, now US. Pat. No. 3,542,952filed May 18, 1967.

BACKGROUND OF THE INVENTION In many electronic environments it isnecessary to sense current changes and convert the current changes tovoltage changes, hence, current-to-voltage conversion systems have foundwidespread use. For example, in many temperature measuring systems athermistor is used to sense temperature changes. The resistance of thethermistor changes with thermal changes to vary the amount of currentpassing through the thermistor. This current change must be amplifiedbefore it can be measured by a measuring instrument. However, mostamplifying circuits are voltage amplifiers. Therefore, it is necessaryto use a current-to-voltage system to convert the thermistor currentchanges to voltage changes. The voltage changes are then amplified anddetected in a voltage measuring instrument.

While current-to-voltage conversion systems have found widespread use,their operation has not always been entirely satisfactory. Specifically,prior art current-to-voltage conversion systems have normally addedadditional resistance to the current side of the conversion system. Theaddition of resistance has decreased the sensitivity of the conversionsystem and increased the error in the output signal. That is, theaddition of resistance will materially change the characteristics of thesignal being measured. Hence, it is desirable to have a sensing systemthat will not introduce either voltage or resistance to the circuitbeing measured.

Moreover, prior art conversion systems have found it difficult to workwith low level signal changes because the conversion systems haveconverted a steady state current plus the change in current. That is,small flucuations or changes are hard to detect when the measuring ordetection device must detect a large steady state signal plus a smallchange.

Therefore, it is an object of this invention to provide a new and simpleeurrent-to-voltage conversion system with an inherent accuracy notachievable before with simple circuitry.

It is also the object of this invention to provide a new and improvedcurrent-to-voltage conversion system wherein the system does not addresistance or voltage to the current sensing side of the system.

It is a still further object of this invention to provide a new andimproved current-to-voltage conversion system wherein small currentchanges are converted to large voltage changes.

It is another object of this invention to provide a new and improvedmeasuring system including a novel current-to-voltage conversion systemthat converts current changes without regard to any steady statecurrent.

With the present state of the art in sound reproduction, the onlyserious problem remaining for true reproduction of sound is thedistortion of the CUR- RENT output from the amplifier to the speakercaused by the changing electrical impedance of the speaker (commonlyused, permanent magnet moving coil cone speaker). It is well known thatthe electrical impedance of subject speakers will vary with suchparameters as current level, frequency, motion amplitude, and the basicspeaker design (permanent magnet and coil size).

Assuming a perfect voltage feedback amplifier, it is impossible tocorrect the distortion of the current flow caused by changing speakerimpedances. Prior art amplifier systems can not sense current distortionbecause the output voltage signal will be the same as the input voltagesignal even though radical distortion of the current flow to the speakeris occurring. However, speakers are current controlled devices and conedisplacement is due to the force caused by the current. Thisproblem ofspeaker induced current distortion has long been recognized, but untilnow there has been no adequate solution.

The desired solution is to be able to sense the current going to thespeaker (without introducing any loading and distorting device into thesystem) and then converting this current through proper transformationto a voltage that can be used as feedback. This voltage should have theexact wave form as the current wave form with no phase shift. Hence, theamplifier would now be forced to produce a current that will beindependent of impedance changes in the speaker and will be proportionalto the input voltage signal. The preceding definition can properly becalled a current source amplifier. Until now, this device has never beenachieved.

Therefore, it is a further object of this invention to provide a new andimproved sound reproduction system that reduces distortion in the audiooutput caused by impedance changes in the speaker.

SUMMARY OF THE INVENTION In accordance with a principle of theinvention, a novel current-to-voltage conversion system is used for DCoperation and with a slight modification for AC operation. In DCoperation, a current sensing loop and a current reference loop areconnected to a novel zero junction circuit. The reference current, ofconstant magnitude, is used to buck out most of the sensing loopcurrent. Hence, the difference between these currents is sensed by thezero junction circuit and converted to a proportional voltage.

A zero junction circuit has two points in its circuitry where thevoltage difference is insignificantly small even under changing currentflow. The presence of this junction to any outside circuit connected tothese two points does not affect the current flow in the outsidecircuitry. This device therefore provides a junction able to sensecurrent summations of a circuit that has been connected to the zerojunction points without any adverse effect on the circuit. In fact, manyoutside circuits can be connected to these zero-junction points for atotal summation of all of the current flowing in all of the outsidecircuits. In accordance with the invention the current summations arethen transferred to a voltage amplifying loop which completes thecircuitry. Hence, a current-to-voltage transformation takes placewithout modifying the original monitored signal.

In accordance with another principle of the invention, the zero junctioncircuit is composed of two active elements (transistors). The firstelement (transistor) senses the current differential and the secondelement together with the first provides a zero junction; this providesa non-loading active junction with the capability of achieving currentsummation (algebraically). The current flow differential is sensed bythe zero junction circuit to cause an identical current flowin thevoltage gain loop of the first element. The current flow in this loopacross a resistance provides a voltage linearly related with the currentdifferential sensed by the zero junction circuit.

In accordance with still another principle of the invention, anamplifier is connected to the output of the current-to-voltageconversion system. This amplifier both bucks out a steady state voltageand amplifies the changing output voltage. The amplified voltage isapplied to a voltage measuring instrument, controlling device, orcontrolled device. And, the voltage measuring instrument provides avoltage measurement of the original current changes sensed in thecurrent sensing Cll'CLllL In accordance with a still further principleof the invention, and with slight modification, the current-tovoltageconversion system can be used for AC operation in the audio frequencyrange or even higher.

In accordance with but still another principle of the invention, thecurrent-to-voltage conversion network can be used with a loud speakercurrent transformed to a proper magnitude. This current is fed to thezero junction circuit and converted to a voltage of proper magnitudewhich is used as a feedback voltage in a high gain operational poweramplifier. An inherent characteristic obtained with an amplifier havinga resistive feedback network is that the feedback voltage has the samewave form and has an amplitude that is proportional to the signalvoltage applied to the input of the amplifier. Hence, the current fed tothe loud speaker has the same wave form and proportional amplitude asthe input signal voltage applied to the amplifier.

The foregoing objects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed descriptionwhen taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a common base transistor circuit usedto explain a portion of the invention;

FIG. 2 is a schematic diagram of a current bucking reference and sensingloops used to explain a part of the invention;

FIG. 3 is a schematic diagram of two transistors used to explain thezero junction aspect of the invention;

FIG. 4 is a schematic diagram illustrating one embodiment of theinvention;

FIG. 5 is a schematic diagram illustrating a measuring system made inaccordance with the invention;

FIG. 6 is a partially schematic and partially blocked diagramillustrating a sound reproduction current compensating system made inaccordance with the invention; and

FIG. 7 is a schematic diagram illustrating how the current-to-voltageconversion system of the invention is connected in a sound reproductionsystem.

. I 4 DESCRIPTION OF THEPREF ERRED EMBODIMENTS Turning now to thedrawings wherein like reference numerals designate like parts throughoutthe several views, FIG. 1 is a schematic diagram of a common basetransistor circuit and comprises a first NPN (or PNP properly used)transistor Q1, an emitter resistor R1, a collector resistor R2, a firstvoltage source VI, and a second voltage source V2.

The emitter of Q1 is connected through R1 in series with V1 to the baseof Q1 so that the emitter-base junction is forward biased. The collectorof transistor 01 is connected through R2 in series with V2 to the baseof Q1 so that the collector-base junction is reverse biased.

As is well known in the transistor art, the current in thebase-collector loop of a common base circuit will track the current inthe base-emitter loop. That is, the current conversion factor or a of acommon base circuit for most transistors is near unity. Hence, if thecurrent in the base-emitter loop is made to vary the current in thebase-collector loop follows it in a very near linear manner and becausethe current in that loop tracks the current in the emitter-base loop,the voltage drop across R2 tracks the current in the emitter-base loop.Further, by making R2 much, much larger than R1 and V2 larger than V1 alarge voltage drop occurs across R2. This R2 voltage drop is then themajor voltage drop in the collector-base loop. Hence, the common basecircuit provides a near linear current-to-voltage conversion. Further,by making V2 and R2 large, a small change in emitter-base current causesa large change in the voltage drop across R2.

FIG. 2 is a schematic diagram of a current reference loop and a currentsensing loop connected in bucking relationship. The circuit illustratedin FIG. 2 comprises a third voltage source V3, a fourth voltage sourceV4, a thermistor T and the emitter-base resistor R1 of FIG. 1. ResistorR1 is connected in series with the third voltage source V3 between apair of points A and B. A wire connects points A and B so that a currentreference loop is formed. Also connected across points A and B is thethermistor T in series with the fourth voltage source V4. The wireconnecting points A and B is common to T and V4 so that a thermistorsensing loop is formed. The voltage sources V3 and V4 are connected sothat they apply opposite polarity. voltages to points A and B.

It will be appreciated by those skilled in the art that for a particularvalue of the thermistors resistance the values of R1, V3 and V4 can bechosen so that the voltage across points A and B is zero. When thevoltage across points A and B is zero the current flow through the wireconnecting these points is also zero, i.e. a balance condition exists.Thereafter if the resistance of the thermistor T increases or decreasesan unbalance condition occurs and a current flows in one direction orthe other through the wire connecting points A and B. This current flowis the difference between the current flow in the reference loopcomprising V3, R1, and points A and B and the current in the sensingloop comprising T, V4, and points A and B. That is, FIG. 2 illustrates asimple current bucking network wherein the current through a wireconnecting points A and B is the difference between the currents in thetwo loops connected to those points.

It should be noted that a circuit of the type illustrated in FIG. 2 isconsiderably more accurate for measuring than is a single loop circuit.Specifically, if the wire connecting points A and B is removed and anidea] meter of no resistance is placed across these points, the meter ismuch more sensitive to fluctuations caused by variationsin theresistance of the thermistor, than is the meter if placed in a loopcomprising a meter, the thermistor T, and the voltage source V4. Morespecifically, the meter can be adjusted so that full scale deflection iscaused by the fluctuating current as opposed to full scale deflectionbeing caused by a steady state current plus the fluctuating current.

FIG. 3 is a simplified schematic diagram illustrating two transistorsconnected to provide zero voltage across their emitter terminals. Thecircuit illustrated in FIG. 3 comprises a first NPN transistor Q1 and asecond NPN transistor Q2. The first transistor Q1 is illustrated ashaving its emitter connected to point A and the second transistor Q2 isillustrated as having its emitter connected to point B. The bases of thetransistors are connected together and to a common wire. For purposes ofclarity, the collectors of the transistors are not illustrated.

By connecting the common base wire to a positive voltage source and bysuitably adjusting the transistors biasing resistors (not shown) a nearequal amount of current flow will pass through the transistors.Reasonable changes of current that would produce an unbalance in thebase-emitter flow of Q1 and Q2, will not affect the near zero-voltagedifferential between points A and B.

As will be hereinafter described, this transistor network can beconnected across points A and B of FIG. 2 to sense the unbalancecurrent. More specifically, a variation in current flow through thetransistors is a variation in the emitter-base current. Thisemitter-base current variation is sensed in a common base circuit of thetype illustrated in FIG. 1. Hence, the dual transistors of FIG. 3provide a zero junction network. Further, the common base conversioncircuit in FIG. 1 provides isolation between the sensing emitter-baseloop and the collector-base loop.

Turning now to FIG. 4, which illustrates the foregoing relationship;that is, FIG. 4 illustrates the systems described in FIGS. 1, 2 and 3connected together.

The system illustrated in FIG. 4 comprises the pair of NPN transistorsQ1 and Q2, the reference loop comprising resistor R1 and voltage sourceV3, the thermistor loop comprising thermistor T and voltage source V4,the junction between R1 and T or point A connected to the emitter of Q1,and the junction between V3 and V4 or point B connected to the emitterof Q2. The bases of Q1 and Q2 are connected together and through a thirdresistor R3 to the positive side of the third voltage source V3. Thecollector of O1 is connected to the base of Q1 through the seriescircuit consisting of the second voltage source V2 and the secondresistor R2. Finally, the collector of Q2 is connected to the base of Q2through a fourth resistor R4. A pair of output terminals are connectedacross the voltage sensing resistor R2.

The circuit illustrated in FIG. 4 combines the operations of thecircuits illustrated in FIGS. 1, 2 and 3 to provide a current-to-voltageconversion system. That is, the components of the circuit are picked sothat the voltage across points A to B is near zero for any desiredresistance value of the thermistor T to provide an initial or steadystate condition.

While there is zero voltage across points A and B, there is currentflowing through the transistors due to the bias voltages provided by thevarious voltage sources. The bias current through the emitter-base loopof Q1 creates a current in its collector-base loop to develop an initialor steady state voltage across the output terminals.

When the thermistors resistance varies the current flow through theemitter-base junction of Q1 varies. As described with respect to FIG. 1,the variation in the emitter-base current flow of Q1 varies the currentin 01 s collector-base circuit. This latter current variation varies thevoltage drop across R2 thereby varying the voltage across outputterminals. Hence, the overall system provides a current-to-voltageconversion. Further, by making V2 and R2 large a very small currentfluctuation through the emitter-base circuit of Q1 provides a largevoltage fluctuation across R2. That is, the circuit components can bechosen so that a micro or milliamp current change through theemitter-base circuit of Q1 provides a change across R2 measured inmagnitude of several volts.

It will be appreciated by those skilled in the art and others that theoutput voltage of the embodiment of the invention illustrated in FIG. 4is equal to steady state voltage E plus a voltage change A E caused by achange in the current flowing through the emitter base circuit of Q1.While this condition may be satisfactory in some environments it may notbe satisfactory in other environments. Specifically, in someenvironments it may be preferable to measure only A E not E A E; this isaccomplished by the circuit illustrated in FIG. 5.

The circuit illustrated in FIG. 5 comprises the circuit illustrated inFIG. 4 plus an amplifier having a voltage gain of one that bucks out thesteady state voltage E.

The additional components in the circuit illustrated in FIG. 5 comprisea third NPN transistor Q3, a fourth NPN transistor Q4, a first PNPtransistor Q5, a fifth resistor R5, a sixth resistor R6, a seventhresistor R7, an eighth resistor R8, a ninth resistor R9, a meter M, anda fifth voltage source V5.

Transistors Q3 and Q4 are connected in a differential amplifierarrangement. That is, the emitters of Q3 and Q4 are connected togetherand through the eighth resistor R8 to the negative side of the secondvoltage source V2. The base of Q3 is connected to the bases of Q1 and Q2and the base of Q4 is connected to one end of the ninth resistor R9. Theother end of R9 is connected to the negative side of V2. The collectorof Q3 is connected through the sixth resistor R6 in series with thefifth resistor R5 to the positive side of V2 and the collector of O4 isconnected through the seventh resistor to the positive side of V2. Thejunction between R5 and R6 is connected to the base of Q5 and theemitter of O5 is connected to the positive side of V2. The collector ofQ5 is connected to the base of Q4 and through the meter M to thepositive side of the fifth voltage source V5. The negative side of V5 isconnected to the negative side of V2.

The amplifier of FIG. 5 bucks out the steady state voltage across R2 andapplied only the change in voltage caused by the change in currenthereinabove described to pass through the meter M. The differentialamplifier formed by Q3, Q4 and Q5 provides this relationship with powergain and prevents circuit loading. That is, V5 applies a voltage thatbucks out the steady state voltage at the collector of Q5 and, thus,only the actual change in voltage caused by the thermistor T change incurrent is applied to Q5s output and in turn applied to the meter M.Hence, the overall system provides a measuring system having a directrelationship between the current change through thermistor T and thevoltage change across the meter M. This system can be designed so thatzero through full-scale deflection occurs for the variations in currentflow through the thermistor. The thermistor can be used to measuretemperature changes in a conventional manner. Or, the thermistor can beused to sense other changes that are represented by heat changes, suchas a device for measuring the efficiency of combustion by sensing theheat emitted by a chemical reaction, for example. Further, thethermistor is only by Way of example, it can be replaced by othercurrent sensing elements such as a photodecting device having aresistance that changes with the amount of light impinging on itsphotosensitive surface to vary the amount of current flowing through it,for example.

F IG. 6 and FIG. 7 illustrate a further embodiment of the inventionwherein the current-to-voltage conversion system is used to sense thecurrent passing through a speaker coil and generates a feedback voltageto be applied to the speakers amplifier. The system illustrated in FIG.6 comprises an amplifier A, a speaker S, a toroidal transformer T1, thecurrent-to-voltage conversion system X, a first capacitor C 1, a tenthresistor R10, a second capacitor C2, and an eleventh resistor R1 1.

The output of the amplifier A is connected to the primary winding of T1in series with the speakers winding. The secondary winding of T1 isconnected to the input of the current-to-voltage conversion system X.The transformer takes the place of the thermistor illustrated in FIGS. 4and 5. The output of the current-tovoltage conversion system X isconnected through the tenth resistor R10 in series with the firstcapacitor C1 across the input of the amplifier A. A coupling networkcomprising C2 and R11 is illustrated as connecting a pair of inputterminals to the amplifier A.

FIG. 7 illustrates that the bucking circuit has been eliminated alongwith the thermistor circuit and that the current transformer has beenadded, therefore. This current transformer transforms high currentlevels to small usable current levels preferably in the range of 0 to 10ma.

The current-to-voltage conversion system X illustrated in FIG. 7comprises the first and second NPN transistors 01 and Q2, first, secondthird, fourth, fifth and sixth resistors R12, R13, R14, R15, R16 andR17, and a voltage source V6. R12 equals R13 for the preferred operationof the system. The emitter of the first transistor is connected to pointA and through the first resistor R12 to the positive terminal of thevoltage source V6. Similarly, the emitter of the second transistor O2 isconnected to point B and through the second resistor R13 to the positiveside of V6. The third and fourth resistors R14 and R15 are connected inseries across the voltage source V6 and the junction between the thirdand fourth resistors is connected to the bases of the first and secondtransistors. The fifth resistor R16 is connected between the collectorof Q1 and the negative side of V6 and the sixth resistor R17 isconnected between the collector of Q2 and the negative side of V6.

The secondary of the transformer is applied across the points A and B,which comprise the points of the zero junction circuitry. Hence, thetransformer is electrically shorted in its output. It is well known factthat the output of a current transformer must be shorted in order'toproduce a near perfect current transformation of the wave form seen inthe primary windings. Also, under these circumstances there is no phaseshift. Hence, current must be sensed or detected with a zero junctioncircuit in order that the secondary will be short circuited and properperformance given. This allows a monitoring of the current flow in thesame fashion as the zero junction circuit has worked in the previousexplanation.

In operation, the transformer T1 senses current flow patterns in thesignal applied to the speaker S. These current flow patterns aretransformed by the currentto-voltage conversion system X in the mannerhereinabove described to voltage patterns that duplicate proportionallythe current flow patterns. These voltage flow patterns are connectedthrough the capacitor C1 and resistor R10 to the input of the amplifierA- to correct for the distortions in the current flow pattern producedby the speaker impedance changes. That is, the voltage fluctuationsapplied to the input of the amplifier operate to negate any distortionin the current flow pattern thereby preventing distortion in thespeakers audio output. Hence, the system improves the performance of thespeaker in an audio sound system by eliminating current flow patterndistortion caused by changing speaker impedance.

The foregoing has described preferred embodiments of the invention. Thatis, FIG. 5 illustrates a measuring system utilizing thecurrent-to-voltage conversion system of the invention and FIG. 6illustrates an apparatus for improving the speaker performance of anaudio system by using the current source amplifier previously described.Hence, the electrical impedance of the speaker can change in any knownway without affecting the current-force relationship needed to producehigh-fidelity sound. In summary, this invention eliminates impedancechange as a distorting factor, something which has not been achieved inprior art sound reproduction systems.

Further, in light of the foregoing disclosure of the invention, itshould be apparent to a person skilled in the art that the subjectmatter of this invention can be used to algebraically sum two or more ACsignals without cross modulation. For example, in making a recording, asignal carrying a singers voice can be combined with a signal carryingthe orchestra all without cross modulation.

The current-to-voltage conversion system of the invention can beutilized in other measuring systems or in other types of audio soundsystems. Moreover, the invention can be used with force coil operatingdevices in other than audio sound systems such as recording instrumentsystems, for example. In addition, the currentto-voltage conversionsystem is suitable for use in other environments where it is desired toaccurately convert current-to-voltage for the purpose of measurement andcontrol. This invention is particularly suitable in environments wherethe current is low but the desired voltage must be high. Because a highvoltage output from a small current input is achieved by the invention,unsophisticated amplifying devices can be used to amplify the voltagechanges to even higher voltages and/or power levels, if necessary. Thatis, many prior art current-to-voltage conversion systems change milliampchanges to millivolt changes, for example. However, this inventionprovides a means for changing a micro or milliamp change to full voltchanges. Hence, the conversion ratio is considerably improved. Becauseof this direct transformation to a relatively high voltage,amplification of the output voltage can be more easily performed.

What is claimed is:

1. An electrical measuring network comprising:

a sensing branch;

a reference branch;

a zero junction branch, said sensing branch, said reference branch andsaid zero junction branch all being connected in parallel, said zerojunction branch comprising:

i. a first circuit, including a first active circuit element, connectedso as to detect the current difference between the sensing branch andthe reference branch and converting said current difference into avoltage; and,

ii. a second circuit, including a second active circuit element, forproducing a voltage that is essentially equal to, but opposite in Signfrom, the voltage drop across said first circuit, said first and secondcircuits being connected together so that the combined voltage dropacross said zero junction branch is essentially zero; and,

a measuring device connected to said first circuit so as to measure saidvoltage.

2. An electrical measuring network as claimed in claim 1 including anamplifier connected between said first circuit and said measuring devicefor negating the effect of a steady state current.

3. An electrical measuring network as claimed in claim 2 wherein saidfirst active circuit element is a first transistor of one polarity andwherein said second active circuit element is a second transistor ofopposite polarity to said first transistor.

4. An electrical measuring network as claimed in claim 3 wherein saidsensing branch includes a sensing element and a first voltage source andwherein said reference branch includes a first resistor and a secondvoltage source.

5. An electrical measuring circuit as claimed in claim 4 wherein saidfirst resistor is connected in series with said second voltage sourcebetween the emitters of said first and second transistors and whereinsaid sensing element and said first voltage source are also connected inseries between the emitters of said first and second transistors, saidfirst and second voltage sources having oppositely connected poles.

6. An electrical measuring network as claimed in claim 5 including:

a third voltage source; a second reslstor, said second resistor beingconnected in series with said third voltage source between thebase-collector terminals of said first transistor;

a third resistor, said third resistor being connected between the basesof said first and second transistors and the junction between said firstresistor and second voltage source; and,

a fourth resistor, said forth resistor being connected between thebase-collector terminals of second transistor.

7. An electrical measuring network as claimed in claim 6 wherein saidamplifier is a difference amplifier having a reference voltage tocompensate for the steady state voltage of said zero junction branch.

8. An electrical measuring circuit as claimed in claim 7 wherein saidsensing means is a thermistor.

1. An electrical measuring network comprising: a sensing branch; areference branch; a zero junction branch, said sensing branch, saidreference branch and said zero junction branch all being connected inparallel, said zero junction branch comprising: i. a first circuit,including a first active circuit element, connected so as to detect thecurrent difference between the sensing branch and the reference branchand converting said current difference into a voltage; and, ii. a secondcircuit, including a second active circuit element, for producing avoltage that is essentially equal to, but opposite in sign from, thevoltage drop across said first circuit, said first and second circuitsbeing connected together so that the combined voltage drop across saidzero junction branch is essentially zero; and, a measuring deviceconnected to said first circuit so as to measure said voltage.
 2. Anelectrical measuring network as claimed in claim 1 including anamplifier connected between said first circuit and said measuring devicefor negating the effect of a steady state current.
 3. An electricalmeasuring network as claimed in claim 2 wherein said first activecircuit element is a first transistor of one polarity and wherein saidsecond active circuit element is a second transistor of oppositepolarity to said first transistor.
 4. An electrical measuring network asclaimed in claim 3 wherein said sensing branch includes a sensingelement and a first voltage source and wherein said reference branchincludes a first resistor and a second voltage source.
 5. An electricalmeasuring circuit as claimed in claim 4 wherein said first resistor isconnected in series with said second voltage source between the emittersof said first and second transistors and wherein said sensing elementand said first voltage source are also connected in series between theemitters of said first and second transistors, said first and secondvoltage sources having oppositely connected poles.
 6. An electricalmeasuring network as claimed in claim 5 including: a third voltagesource; a second resistor, said second resistor being connected inseries with said third voltage source between the base-collectorterminals of said first transistor; a third resistor, said thirdresistor being connected between the bases of said first and secondtransistors and the junction between said first resistor and secondvoltage source; and, a fourth resistor, said forth resistor beingconnected between the base-collector terminals of second transistor. 7.An electrical measuring network as claimed in claim 6 wherein saidamplifier is a difference amplifier having a reference voltage tocompensate for the steady state voltage of said zero junction branch. 8.An electrical measuriNg circuit as claimed in claim 7 wherein saidsensing means is a thermistor.